1. Field of the Invention
The present invention relates to a structure of a motor and more particularly to a structure of a motor whose performance is bettered by improving its magnetic circuit.
2. Description of the Prior Art
Stepping motors are widely used in office automation equipment, computer peripheral equipment, industrial equipment, and the like. The stepping motor is an electric motor rotatably driven by an input pulsating alternating current. The angle of rotation is proportional to the number of pulses of the input alternating current and the rotational speed is proportional to the frequency of the same. Accordingly, there is no need for detecting the rotor position of the stepping motor. Namely, since the stepping motor is driven in an open loop system, the rotor rotates such that the magnetic reluctance is minimized. Hence, the rotating condition of the rotor depends on both the magnetic flux generated from the rotor (magnetic flux distribution of the magnet) and the magnetic flux generated from the stator (magnetic flux distribution of the armature).
The shape of the stator pole tooth as the interface between the rotor and the stator, especially the shape of the stator pole tooth of a claw pole type, cannot be formed to be considerably long on account of technical difficulty in manufacturing. When it is formed to be long, the pole tooth is formed in the shape of a trapezoid having a wider base to prevent magnetic saturation from occurring in the pole tooth. Further, the stator pole teeth forming one phase are made in the same shape. Hence, the stepping motor has a defect that, when it is driven, its rotational fluctuation becomes so great as to produce vibration.
In modern high performance equipment, there are increasing possibilities that the vibration occurring in the stepping motor adversely affects other component parts and causes malfunctions in the equipment. Therefore, as methods for suppressing the motor vibration, there are used a method for keeping the stepping motor in a floating condition by using vibration-proofing rubber as disclosed in the gazette of Japanese Patent Laid-open No. Hei 4-26355 and a method to decrease vibration by modifying the drive circuit as disclosed in the gazette of Japanese Utility Model Laid-open No. Sho 59-111500. These methods were unavoidably accompanied by increase in the number of component parts and complexity of circuits and, hence, invited increases in cost. Further, these methods were countermeasures taken from outside the stepping motor leaving the source of the vibration still existing in the body of the stepping motor and, therefore, they were not essential solutions of the problem. Other motors than stepping motors having magnetic poles, such as DC servomotors and brushless motors, also have similar problem.
In order to obtain a low-vibration motor by cutting off the source of the vibration for suppressing the vibration of the motor itself, it is an effective way to lower the detent torque. Although the detent torque can be lowered by increasing the gap between the rotor and the pole tooth, as a general practice for lowering the detent torque, it unavoidably follows lowering of the dynamic torque and, thereby, the performance of the stepping motor is deteriorated. Therefore, it becomes necessary to advance analysis of the detent torque as the essential vibration source and to achieve low vibration of the motor without deteriorating its performance by making changes in the structure of the motor.